Redis persistence RDB

Redis Is a memory database , If you do not save the database state in memory to disk , So once the server process exits , The database state in the server also disappears , therefore Redis Provides persistence

RDB（Redis DataBase）

What is? RDB

Write the data set snapshot in memory to disk within the specified time interval , That's what jargon says Snapshot snapshot , It will recover soon Read the file directly into memory .

Redis Will create... Separately （fork） A subprocess to persist , Write the data to a temporary file first , Process to be persisted It's all over , Replace the last persistent file with this temporary file . The whole process , The main process is not going to do anything IO Operation of the . This ensures extremely high performance . If large-scale data recovery is needed , And it's not very sensitive to the integrity of data recovery , that RDB The way is better than AOF It's more efficient .RDB The disadvantage is that the data may be lost after the last persistence .

Fork

Fork Its function is to copy the same process as the current process . All the data for the new process （ Variable , environment variable , Program counter, etc ） The values are consistent with the original process , But it's a whole new process , And as a child of the original process .

rdb The saved file is dump.rdb

Configuration location and location SNAPSHOTTING analysis  

The trigger condition mechanism here , We can modify and test ：

save 120 10 # 120 Seconds to modify 10 Times trigger RDB  

Default ：

1 Changed in minutes 1 Ten thousand times

5 Changed in minutes 10 Time

15 Changed in minutes 1 Time

If you want to disable RDB Persistent strategy , Just don't set anything save Instructions , Or to save You can also pass in an empty string parameter .

To complete the modification, it needs to take effect immediately , It can be used manually save command ！ Take effect immediately

The rest of the commands are parsed

Stop-writes-on-bgsave-error： If configured as no, Show that you don't care about data inconsistency or other means of discovery and control , The default is yes.

rbdcompression： For snapshots stored on disk , You can set whether to compress storage . If so ,redis Will be used

LZF Algorithm for compression , If you don't want to consume CPU To compress , It can be set to turn off this function .

rdbchecksum： After storing the snapshot , Can also let redis Use CRC64 Algorithm to check data , But doing so will add about

10% Performance consumption of , If you want to get the maximum performance improvement , You can turn it off . The default is yes.

How to trigger RDB snapshot

1、 The default snapshot configuration in the configuration file , It is recommended to use one more machine as a backup , A copy of dump.rdb

2、 command save Or is it bgsave

• save Just keep it , Whatever else , All blocked
• bgsave,Redis The snapshot operation will be performed asynchronously in the background , The snapshot can also respond to client requests . Can pass lastsave Command gets the last time the snapshot was executed successfully .

3、 perform flflushall command , There will be dump.rdb file , But it's empty , meaningless !

4、 When you exit, it will also produce dump.rdb file ！

How to restore

1、 Will back up files （dump.rdb） Move to redis Install the directory and start the service

2、CONFIG GET dir Get directory

127.0.0.1:6379> config get dir
1) "dir"
2) "/www/server/redis"
 

pros and cons

advantage ：

1、 Suitable for large-scale data recovery

2、 The requirements for data integrity and consistency are not high

shortcoming ：

1、 Make a backup at regular intervals , So if redis accident down If you drop it , All changes since the last snapshot will be lost

2、Fork When , The data in memory is cloned , roughly 2 Double expansion needs to be considered .

Summary